1. Field of the Invention
The present invention relates to a precision arm mechanism suitable for positioning various objects such as semiconductor wafers, cassettes holding such wafers, panels, computer hard discs, and the like for processing and/or use, and specifically for manipulating such objects in a isolated environment such as a vacuum.
2. Description of the Related Art
The use of robot arms for positioning and placing objects is well known. Generally, the arms have Z, R and .theta. motion as defined in a conventional cylindrical coordinate system. The capability of providing straight line motion is very important in the processing of semiconductor wafers so as to allow them to be very accurately positioned at a work station where processing steps take place. The R or straight line radial movement of the end effector or mechanical hand at the end of the arm has been accomplished in a number of manners.
Any number of robot arm designs are utilized in the industry. However, these arms have difficulty dealing with the problem of angular misalignment of the object to be manipulated.
As one example, telescoping arms have been utilized for manipulating objects. In such a structure one slidable member fits within another thus allowing linear extension of the arm.
More commonly, two link arms with equal length links have been utilized for this purpose. The links are connected to each other so that the distal end of the first link is pivotally attached to the proximal end of the second link. The links utilize belt drives which are provided for coordinately rotating the second link to the first link to provide a rotation ratio suitable for manipulating the arms in a desired fashion.
It is also known to utilize an isosceles triangle type linkage wherein two equal length links are pivoted together and a mechanical hand is pivoted to the distal end of the distal link. Pulleys and belts are utilized in such a manner that the angle between the two links changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted.
In previously mentioned U.S. Pat. No. 5,064,340, which is incorporated herein in its entirety by reference, an arm structure is disclosed comprising first, second and third longitudinally extending links each having proximal and distal end portions.
U.S. Pat. No. 5,007,784, which is incorporated herein in its entirety by reference, provides a robotic arm which comprises an end effector structure which has a central portion and two substantially oppositely extending hands each capable of picking up a workpiece.
One limitation of all the foregoing prior art arms is their limitation to a single Z axis on rotation at the bend of the arm mounted to an elevator or other mounting platform. As noted above, a very important problem which exists with present day robotic arm mechanisms is that they can only follow a radial straight line (R) path or a circular (.theta.) path in the R, .theta. plane from one point to another. Accordingly, if there is an object to be picked up and moved which is located in a cassette, e.g., a wafer cassette, or at a work station, the arm must first be extended radially into the cassette or work station where it picks up the object, generally by application of a vacuum, then withdrawn radially from the cassette or work station, then rotated to opposite another cassette or work station and then advanced radially into the other cassette or work station where it deposits the object. Also, if there is an obstacle which protrudes into that plane of operation of the arm as may occur in a semiconductor processing operation, an inefficient path must be followed to anything hidden behind or shadowed by that obstacle, namely, a straight line path must be followed to withdraw the arm radially inwardly beyond the obstacle and then radial motion must be imparted to move the end effector of the arm past the obstacle followed by radial outward motion to the desired work station. The ability to follow a curved path would be desirable in that it would allow faster operation of the robotic arm mechanism.
A particular problem which occurs with flat display panels is that they are often present in cassettes with a certain degree of angular and linear misalignment. Such panels must be aligned properly at a work station. With conventional robotic arms this must be accomplished by placing the panel on a chuck, which has sensors, e.g., CCD sensors, which detect the misalignment, using the chuck to rotate the panel, and then picking it up and transporting it to the work station in proper alignment. This is so since with conventional robotic arms the arm cannot be rotated with respect to the panel (or wafer) coordinate frame which does not coincide (due to the position and angular misalignment) with the coordinate frame of the end effector. The conventional arms can move only along the longitudinal axis of the end effector, and to rotate it so it cannot compensate for the misalignment without intermediately being dropped onto a pin or the like. It would be highly desirable if the desired alignment could be attained without the intermediate use of such a chuck or pin. In a similar manner, it would be highly desirable if it was possible to align wafers about their geometric centers without utilizing such a chuck or pin.
Another problem which is common with both wafers and panels which are loaded from cassettes or picked up at work stations is that the Z axis of the robotic arm may not be completely parallel to the axis of the cassette, etc. due to alignment errors. The relative tilt may be in any direction and is usually only a few degrees. When this misalignment is present the arm cannot properly approach and pick up the wafers/panels with the end effector properly aligned whereby the exact positioning of the wafers/panels is not fully controllable. Previously filed U.S. patent application Ser. Nos. 08/661,292, filed Jun. 13, 1996, and application Ser. No. 08/788,898, filed Jan. 23, 1997, each of which is incorporated herein in its entirety, disclose robotic arms having a universally tiltable Z-axis.
With prior art robotic arms for processing semiconductor wafers and flat panels the cassettes and work stations must generally be aligned so as to be entered by the end effectors of the arms with the end effector moving radially in a straight line. It is not, for example, possible to enter the cassettes or workstations if their longitudinal axes are not parallel to the Z axis. Yet, since space is at a premium in a wafer or flat panel processing operation it would be highly desirable to be able to so arrange cassettes and/or workstations.
Typically such robotic arms are useful in the processing of semiconductor wafers wherein the production occurs in a vacuum or isolated environment. The movement of the robot occurs in the vacuum, but the arm must be mounted on an elevator structure, such as that disclosed in co-pending application Ser. No. 08/788,898 in order to provide greater degrees of freedom to the arm. However, in embodiments providing robots with a universally tiltable Z axis, no provision has heretofore been made for operation and isolation of the robot arm in a vacuum.